High electron mobility transistor and method of forming the same

ABSTRACT

A high electron mobility transistor (HEMT) includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and is different from the first III-V compound layer in composition. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are disposed on the second III-V compound layer. Each of the source feature and the drain feature comprises a corresponding intermetallic compound at least partially embedded in the second III-V compound layer. Each intermetallic compound is free of Au and comprises Al, Ti or Cu. A p-type layer is disposed on a portion of the second III-V compound layer between the source feature and the drain feature. A gate electrode is disposed on the p-type layer. A depletion region is disposed in the carrier channel and under the gate electrode.

TECHNICAL FIELD

This disclosure relates generally to a semiconductor structure and, moreparticularly, to a high electron mobility transistor (HEMT) and methodfor forming a high electron mobility transistor.

BACKGROUND

In semiconductor technology, due to their characteristics, GroupIII-Group V (or III-V) semiconductor compounds are used to form variousintegrated circuit devices, such as high power field-effect transistors,high frequency transistors, or high electron mobility transistors(HEMTs). A HEMT is a field effect transistor incorporating a junctionbetween two materials with different band gaps (i.e., a heterojunction)as the channel instead of a doped region, as is generally the case formetal oxide semiconductor field effect transistors (MOSFETs). Incontrast with MOSFETs, HEMTs have a number of attractive propertiesincluding high electron mobility and the ability to transmit signals athigh frequencies, etc.

From an application point of view, enhancement-mode (E-mode) HEMTs havemany advantages. E-mode HEMTs allow elimination of negative-polarityvoltage supply, and, therefore, reduction of the circuit complexity andcost. Despite the attractive properties noted above, a number ofchallenges exist in connection with developing III-V semiconductorcompound-based devices. Various techniques directed at configurationsand materials of these III-V semiconductor compounds have beenimplemented to try and further improve transistor device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure may be understood from the followingdetailed description and the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view of a semiconductor structure having ahigh electron mobility transistor (HEMT) according to one or moreembodiments of this disclosure.

FIG. 2A shows a band gap diagram along a comparative HEMT.

FIG. 2B shows a band gap diagram along the HEMT shown in FIG. 1.

FIG. 3 is a flowchart of a method of forming a semiconductor structurehaving a HEMT according to one or more embodiments of this disclosure.

FIGS. 4 to 11 are cross-sectional views of a semiconductor structurehaving a HEMT at various stages of manufacture according to one or moreembodiments of the method of FIG. 3.

FIGS. 12 to 14 are cross-sectional views of a semiconductor structurehaving a HEMT at various stages of manufacture according to one or moreembodiments of the method of FIG. 3.

DETAILED DESCRIPTION

The making and using of illustrative embodiments are discussed in detailbelow. It should be appreciated, however, that the disclosure providesmany applicable inventive concepts that can be embodied in a widevariety of specific contexts. The specific embodiments discussed aremerely illustrative and do not limit the scope of the disclosure.

A plurality of semiconductor chip regions is marked on the substrate byscribe lines between the chip regions. The substrate will go through avariety of cleaning, layering, patterning, etching and doping steps toform integrated circuits. The term “substrate” herein generally refersto the bulk substrate on which various layers and device structures areformed. In some embodiments, the bulk substrate includes silicon or acompound semiconductor, such as GaAs, InP, Si/Ge, or SiC. Examples ofsuch layers include dielectric layers, doped layers, polysilicon layersor conductive layers. Examples of device structures include transistors,resistors, and/or capacitors, which may be interconnected through aninterconnect layer to additional integrated circuits.

FIG. 1 is a cross-sectional view of a semiconductor structure 100 havinga high electron mobility transistor (HEMT) according to one or moreembodiments of this disclosure.

Referring to FIG. 1, the semiconductor structure 100 having a HEMT isillustrated. The semiconductor structure 100 includes a substrate 102.In the present example, the substrate 102 includes a silicon substrate.In some embodiments, the substrate 102 includes a silicon carbide (SiC)substrate or sapphire substrate.

The semiconductor structure 100 also includes a heterojunction formedbetween two different semiconductor material layers, such as materiallayers with different band gaps. For example, the semiconductorstructure 100 includes a non-doped narrow-band gap channel layer and awide-band gap n-type donor-supply layer. In at least one embodiment, thesemiconductor structure 100 includes a first III-V compound layer (orreferred to as a channel layer) 104 formed on the substrate 102 and asecond III-V compound layer (or referred to as a donor-supply layer) 106formed on the channel layer 104. The channel layer 104 and thedonor-supply layer 106 are compounds made from the III-V groups in theperiodic table of elements. However, the channel layer 104 and thedonor-supply layer 106 are different from each other in composition. Thechannel layer 104 is undoped or unintentionally doped (UID). In thepresent example of the semiconductor structure 100, the channel layer104 includes a gallium nitride (GaN) layer (also referred to as the GaNlayer 104). The donor-supply layer 106 includes an aluminum nitride(AlN) layer (also referred to as AlN layer 106). The GaN layer 104 andAlN layer 106 directly contact each other. In some embodiments, thechannel layer 104 includes a GaAs layer or InP layer. In someembodiments, the donor-supply layer 106 includes an AlGaAs layer, AlGaNor AlInP layer.

A band gap discontinuity exists between the AlN layer 106 and the GaNlayer 104. The electrons from a piezoelectric effect in the AlN layer106 drop into the GaN layer 104, creating a very thin layer 108 ofhighly mobile conducting electrons in the GaN layer 104. This thin layer108 is also referred to as a two-dimensional electron gas (2-DEG), andforms a carrier channel (also referred to as the carrier channel 108).The thin layer 108 of 2-DEG is located at an interface of the AlN layer106 and the GaN layer 104. Thus, the carrier channel has high electronmobility because the GaN layer 104 is undoped or unintentionally doped,and the electrons can move freely without collision or withsubstantially reduced collisions with impurities.

The GaN layer 104 is undoped. Alternatively, the GaN layer 104 isunintentionally doped, such as lightly doped with n-type dopants due toa precursor used to form the GaN layer 104. In at least one example, theGaN layer 104 has a thickness in a range from about 0.5 microns to about10 microns.

The AlN layer 106 is intentionally doped. In at least one example, theAlN layer 106 has a thickness in a range from about 2 nanometers (nm) toabout 20 nm. Compared to other donor-supply layers such as AlGaAs layer,AlGaN layer or AlInP layer, the AlN layer 106 has a larger latticemismatch with the GaN layer 104 than other donor-supply layers with theGaN layer. Hence, the AlN layer 106 could use a relative thin thicknessto create the same concentration of 2-DEG in the carrier channel 108 asother donor-supply layers do. The thinner AlN layer helps the laterformed gate electrode closer to the carrier channel 108 and improves thecapability of gate control. Also, the AlN layer could reduce theelectron scattering effect in the carrier channel 108 and make carriermobility higher.

The semiconductor structure 100 also includes a dielectric cap layer 110disposed on a top surface 107 of the AlN layer 106. The dielectric caplayer 110 further includes a plurality of openings that expose a portionof the AlN layer 106 for a gate electrode formation and source/drainfeatures formation. The dielectric cap layer 110 protects the underlyingAlN layer 106 from damage in the following process having plasma.

The semiconductor structure 100 also includes a source feature and adrain feature disposed on the second III-V compound layer 106 (e.g. AlNlayer 106) and configured to electrically connect to the carrier channel108. The second III-V compound layer 106 has a substantially flat topsurface between the source feature and the drain feature. Each of thesource feature and the drain feature comprises a correspondingintermetallic compound 112. In at least one example, the intermetalliccompound 112 is free of Au and comprises Al, Ti, or Cu. In at leastanother example, the intermetallic compound 112 is free of Au andcomprises AlN, TiN, Al₃Ti or AlTi₂N.

In at least one embodiment, the intermetallic compound 112 is formed inthe openings of the dielectric cap layer 110, at least partiallyembedded in the AlN layer 106 and overlies a portion of the dielectriccap layer 110. Thereby, the intermetallic compound 112 has a non-flattop surface. The intermetallic compound 112 has a top width W_(T) and abottom width W_(B). The top width W_(T) is wider than the bottom widthW_(B).

In some embodiments, intermetallic compound 112 is partially embedded inthe AlN layer 106 and does not overlie a portion of the dielectric caplayer 110. The top width W_(T) and the bottom width W_(B) aresubstantially the same.

In some embodiments, the intermetallic compound 112 is at leastpartially embedded in the AlN layer 106 and a top portion of the GaNlayer 104. In some embodiments, the intermetallic compound 112 is formedby constructing a patterned metal layer in a recess of the AlN layer106. Then, a thermal annealing process is applied to the patterned metallayer such that the metal layer, the AlN layer 106 and the GaN layer 104react to form the intermetallic compound 112. The intermetallic compound112 contacts the carrier channel 108 located at the interface of the AlNlayer 106 and the GaN layer 104. Due to the formation of the recess inAlN layer 106, the metal elements in the intermetallic compound 112diffuse deeper into the AlN layer 106 and the GaN layer 104. Theintermetallic compound 112 improves electrical connection and form ohmiccontacts between the source/drain features and the carrier channel 108.

The semiconductor structure 100 further includes isolation regions 116in the first III-V compound layer 104 and the second III-V compoundlayer 106. The isolation regions 116 isolate the HEMT in the structure100 from other devices in the substrate 102. In at least one example,the isolation region 116 includes a doped region with species of oxygenor nitrogen.

A protection layer 114 is disposed on top surfaces of the dielectric caplayer 110 and the intermetallic compounds 112. The protection layer 114further includes an opening that aligns with an opening in thedielectric cap layer 110. The combined opening of the opening in theprotection layer 114 and the opening in the dielectric cap layer 110exposes a portion of the AlN layer 106 for gate electrode formation. Theprotection layer 114 covers the source feature and the drain feature,and protects the source/drain features from exposure during an annealingprocess in the formation of the isolation regions 116.

The semiconductor structure 100 further includes a p-type layer 120. Thep-type layer 120 is disposed along an interior surface of the combinedopening of the protection layer 114 and the dielectric cap layer 110, onthe exposed portion of the AlN layer 106 and overlying a portion of theprotection layer 114. In some examples, the p-type layer 120 comprisesNiO_(x), ZnO_(x), FeO_(x), SnO_(x), CuAlO₂, CuGaO₂ or SrCu₂O₂. X is in arange of about 1 to about 2. The p-type layer 120 contains pointdefects, for example, ZnO_(x) has Zn interstitials and oxygen vacancies.The point defects generate electron holes and induce p-type conductivityfor the p-type layer 120. The p-type layer 120 depletes the electrons inthe carrier channel 108 under the combined opening. In at least oneexample, the p-type layer 120 has a thickness in a range from about 3 nmto about 30 nm. When the thickness is less than 3 nm, the p-type layercould not deplete the carrier channel 108. It is hard to generate anenhanced-mode HEMT. When the thickness of the p-type layer is largerthan 30 nm, the carrier channel 108 may be completely depleted even ahigh positive gate voltage is applied during operation. It is hard toturn on the carrier channel 108 of this HEMT.

In one embodiment, the p-type layer 120 further includes a plurality ofdopants to induce a density of electron holes in a range of about 10¹⁷to about 10¹⁹ per cm³ within the p-type layer 120. The dopants increasep-type layer 120 toward p-type conductivity and further deplete theelectrons in the carrier channel 108. In some examples, the dopantscomprise phosphorous (P), P₂O₅, arsenic (As) or Zn₃As₂.

The semiconductor structure 100 also includes a gate electrode 124disposed in the combined opening over AlN layer 106 between the sourceand drain features. The gate electrode 124 includes a conductivematerial layer configured for voltage bias and electrical coupling withthe carrier channel 108. In various examples, the conductive materiallayer includes a refractory metal or its compounds, e.g., titanium (Ti),titanium nitride (TiN), titanium tungsten (TiW), titanium tungstennitride (TiWN), tungsten (W) or tungsten nitride (WN). In at leastanother example, the conductive material layer includes nickel (Ni),gold (Au) or copper (Cu). In at least one example, the gate electrode124 is disposed on the p-type layer 120 in the combined opening over theAlN layer 106. The p-type layer 120 not covered by the gate electrode124 is removed to prevent depleting the electrons in the carrier channel108 outside the gate electrode 124 region. Edges of the p-type layer 120and the gate electrode 124 are substantially aligned.

The p-type layer 120 also acts as a gate insulator for thissemiconductor structure 100. The presence of the p-type layer 120between the gate electrode 124 and the donor-supply layer 106 constructsa metal insulator semiconductor high electron mobility transistor(MIS-HEMT). During the operation of the MIS-HEMT, the electrons flow inthe carrier channel 108 between the source feature and the drainfeature. In some embodiments, the electrons inject toward the gateelectrode 124. The p-type layer 120 provides a higher barrier height toprevent the electrons penetrating the p-type layer 120 to the gateelectrode 124. Hence, the p-type layer 120 provides further isolation toprevent gate leakage of the HEMT in the structure 100.

The semiconductor structure 100 also includes a depletion region 122 inthe carrier channel 108 under the combined opening of the protectionlayer 114 and the dielectric cap layer 110. The carrier channel 108becomes normally-off because of the depletion region 122. In theembodiment of FIG. 1, a positive gate voltage should be applied to turnon the carrier channel 108 of this HEMT. In the embodiment of FIG. 1,this HEMT is also called an enhanced-mode HEMT that is opposite to adepletion-mode HEMT. The depletion-mode HEMT has a normally-on carrierchannel and a negative gate voltage is applied to turn off the carrierchannel.

In the above described embodiments, the gate electrode 128, thesource/drain features, and the carrier channel 108 in the GaN layer 104are configured as a transistor. When a voltage is applied to the gatestack, a device current of the transistor is modulated.

FIG. 2A is a band gap diagram along a comparative HEMT having an AlNlayer located directly on the top surface of a GaN layer. Conductanceband E_(c) at an interface 121 of the AlN layer and the GaN layer islower than Fermi level E_(f). A thin layer of 2-DEG appears at theinterface 121 so a normally-on channel is constituted. This conventionalHEMT is a depletion-mode HEMT.

FIG. 2B shows a band gap diagram along the HEMT of the semiconductorstructure 100 shown in FIG. 1. In at least this example of the HEMT ofthe semiconductor structure 100, an AlN layer is on a GaN layer, and ap-type layer is directly on a top surface of the AlN layer. With thepresence of the p-type layer, conductance band E_(c) at the interface(as shown at location 123) of the AlN layer and the GaN layer iselevated in comparison with a comparative HEMT. At the interface (asshown at location 123), the conductance band E_(c) is higher than theFermi level E_(f). The thin layer of 2-DEG disappears at the interfaceso a normally-off channel is constituted. A positive gate voltage isapplied to turn on the carrier channel of this enhanced-mode HEMT.

FIG. 3 is a flowchart of a method 300 of forming a semiconductorstructure having a HEMT according to one or more embodiments of thisdisclosure. Referring now to FIG. 3, the flowchart of the method 300, atoperation 301, a first III-V compound layer is provided. The first III-Vcompound layer is formed on a substrate. Next, the method 300 continueswith operation 302 in which a second III-V compound layer is epitaxiallygrown on the first III-V compound layer. The method 300 continues withoperation 303 in which a source feature and a drain feature are formedon the second III-V compound layer. The method 300 continues withoperation 304 in which a p-type layer is deposited on a portion of thesecond III-V compound layer between the source feature and the drainfeature. The method 300 continues with operation 305 in which a gateelectrode is formed on the p-type layer. It should be noted thatadditional processes may be provided before, during, or after the method300 of FIG. 3.

FIGS. 4 to 11 are cross-sectional views of the semiconductor structure100 having a HEMT at various stages of manufacture according to variousembodiments of the method 300 of FIG. 3. Various figures have beensimplified for a better understanding of the inventive concepts of thepresent disclosure.

Referring to FIG. 4, which is an enlarged cross-sectional view of aportion of a substrate 102 of a semiconductor structure 100 afterperforming operations 301 and 302 in method 300. In some embodiments,the substrate 102 includes a silicon carbide (SiC) substrate, sapphiresubstrate or a silicon substrate. A first III-V compound layer 104, alsoreferred to as a channel layer, is formed on the substrate 102. In theembodiment of FIGS. 4-11, the first III-V compound layer 104 refers to agallium nitride (GaN) layer (also referred to as the GaN layer 104). Insome embodiments, the GaN layer 104 is epitaxially grown by metalorganic vapor phase epitaxy (MOVPE) using gallium-containing precursorand nitrogen-containing precursor. The gallium-containing precursorincludes trimethylgallium (TMG), triethylgallium (TEG), or othersuitable chemical. The nitrogen-containing precursor includes ammonia(NH₃), tertiarybutylamine (TBAm), phenyl hydrazine, or other suitablechemical. In the embodiment of FIGS. 4-11, the GaN layer 104 has athickness in a range from about 0.5 micron to about 10 microns. In otherembodiments, the first III-V compound layer 104 may include a GaAs layeror InP layer.

A second III-V compound layer 106, also referred to as donor-supplylayer, is grown on first III-V compound layer 104. An interface isdefined between the first III-V compound layer 104 and the second III-Vcompound layer 106. A carrier channel 108 of 2-DEG is located at theinterface of the first III-V compound layer 104 and the second III-Vcompound layer 106. In at least one embodiment, the second III-Vcompound layer 106 refers to an aluminum nitride (AlN) layer (alsoreferred to as the AlN layer 106). In the embodiment of FIGS. 4-11, theAlN layer 106 is epitaxially grown on the GaN layer 104 by MOVPE usingaluminum-containing precursor and nitrogen-containing precursor. Thealuminum-containing precursor includes trimethylaluminum (TMA),triethylaluminium (TEA), or other suitable chemical. Thenitrogen-containing precursor includes ammonia (NH₃), tertiarybutylamine(TBAm), phenyl hydrazine, or other suitable chemical. In the embodimentof FIGS. 4-11, the AlN layer 106 has a thickness in a range from about 2nanometers to about 20 nanometers. In other embodiments, the secondIII-V compound layer 106 includes an AlGaAs layer, an AlGaN layer or anAlInP layer.

After performing operations 301 and 302, a dielectric cap layer 110 isdeposited on a top surface 107 of the second III-V compound layer 106.The dielectric cap layer 110 has a thickness in a range from about 100 Åto about 5000 Å. In some embodiments, the dielectric cap layer 110includes SiO₂ or Si₃N₄. In at least one example, the dielectric caplayer 110 is Si₃N₄ and is formed by performing a low pressure chemicalvapor deposition (LPCVD) method without plasma using SiH₄ and NH₃ gases.An operation temperature is in a range of from about 650° C. to about800° C. An operation pressure is in a range of about 0.1 Torr and about1 Torr. The dielectric cap layer 110 protects the underlying secondIII-V compound layer 106 from damage in the following processes havingplasma. Next, two openings 109 in the dielectric cap layer 110 aredefined by lithography and etching processes to expose a portion of thesecond III-V compound layer 106.

Referring back to FIG. 3, method 300 continues with operation 303. FIGS.5 and 6 illustrate cross-sectional views for the manufacture stages forforming the source/drain features.

In FIG. 5, a metal layer is deposited over the dielectric cap layer 110,overfills the openings 109 and contacts the second III-V compound layer106. A photoresist layer (not shown) is formed over a portion of themetal layer and developed to form a feature over the openings 109. Aportion of the metal layer not covered by the feature of the photoresistlayer is removed by a reactive ion etch (RIE) process that etches theexposed portions of the metal layer down to the underlying thedielectric cap layer 110. Metal features 111 are generated by theetching process. The photoresist layer is removed after the formation ofthe metal features 111. The dielectric cap layer 110 protects theunderlying second III-V compound layer 106 from damage during theetching process to form metal features 111. The carriers in carrierchannel 108 of 2-DEG underlying the second III-V compound layer 106 arenot affected during this etching process. The electrical performances ofthe semiconductor structure 100 are positively affected. Therefore, theyield of the overall assembly increases.

In some embodiments, the metal layer of the metal features 111 includesone or more conductive materials. In at least one example, the metallayer is free of gold (Au) and comprises titanium (Ti), titanium nitride(TiN), or aluminum copper (AlCu) alloy. In at least another example, themetal layer includes a bottom Ti/TiN layer, an AlCu layer overlying thebottom Ti/TiN layer, and a top Ti layer overlying the AlCu layer. Theformation methods of the metal layer include atomic layer deposition(ALD) or physical vapor deposition (PVD) processes. Without using Au inthe metal features 111, the method 300 is also implemented in theproduction line of integrated circuits on silicon substrate, because thecontamination concern from the use of Au on the silicon fabricationprocess is eliminated.

In at least one example, the metal feature 111 is formed over theinterior surface of the opening 109 and over a portion of a top surfaceof the dielectric cap layer 110. There is a step height differencebetween the metal feature 111 within the opening 109 and the metalfeature 111 over the dielectric cap layer 110. Hence, the metal feature111 has a non-flat top surface. The metal feature 111 has a top widthW_(T) and a bottom width W_(B). The top width W_(T) is wider than thebottom width W_(B).

In at least another example, metal feature 111 is within the opening 109on the AlN layer 106 and does not overlie a portion of the dielectriccap layer 110. The top width W_(T) and the bottom width W_(B) aresubstantially the same.

FIG. 6 is a cross-sectional view of the semiconductor structure 100after performing a thermal annealing process on the metal features 111.In some embodiments, the thermal annealing process is applied to themetal features 111 such that each metal feature 111, the second III-Vcompound layer 106 and/or the first III-V compound layer 104 react toform an intermetallic compound 112. The intermetallic compound 112 isconfigured as a source/drain feature for effective electrical connectionto the carrier channel 108. As at least one example, a rapid thermalannealing (RTA) apparatus and process are utilized for the thermalannealing. The thermal annealing is operated at an annealing temperaturein a range from about 800° C. to about 1100° C. In at least one example,the intermetallic compound 112 is free of Au and comprises Al, Ti, orCu. In at least another example, the intermetallic compound 112 is freeof Au and comprises AlN, TiN, Al₃Ti or AlTi₂N.

In at least one example, the intermetallic compound 112 is at leastpartially embedded in the AlN layer 106 and over the portion of the topsurface of the dielectric cap layer 110. The step height difference forthe intermetallic compound 112 within the opening 109 and theintermetallic compound 112 over the dielectric cap layer 110 results ina non-flat top surface for the intermetallic compound 112. Theintermetallic compound 112 has a top width W_(T) and a bottom widthW_(B). The top width W_(T) is wider than the bottom width W_(B).

In at least another example, the intermetallic compound 112 is partiallyembedded in the AlN layer 106 and does not overlie the portion of thetop surface of the dielectric cap layer 110. The top width W_(T) and thebottom width W_(B) are substantially the same.

FIG. 7 is a cross-sectional view of the semiconductor structure 100after depositing a protection layer 114 on top surfaces of thedielectric cap layer 110 and the intermetallic compounds 112. In someembodiments, the protection layer 114 includes dielectric materials suchas SiO₂ or Si3N₄. In at least one example, protection layer 114 is Si3N₄and is formed by a plasma enhanced chemical vapor deposition (PECVD)method. The protection layer 116 has a thickness in a range from about100 nanometers to about 700 nanometers.

FIG. 8 illustrates the semiconductor structure 100 after formingisolation regions 116 in the first III-V compound layer 104 and thesecond III-V compound layer 106. The isolation regions 116 isolate theHEMT in the semiconductor structure 100 from other devices in thesubstrate 102. In at least one example, the isolation region 116 isformed by an implantation process with species of oxygen or nitrogen.The protection layer 114 covers the source feature and the drainfeature, and prevents the source/drain features from exposure during anannealing process after the implantation process for the isolationregion 116 formation.

FIG. 9 illustrates the semiconductor structure 100 after forming acombined opening 118 in the protection layer 114 and the dielectric caplayer 110. A patterned mask layer (not shown) is formed on a top surfaceof the protection layer 114 and an etching process is performed toremove a portion of the protection layer 114 and the dielectric caplayer 110. The opening 118 exposes a portion of the top surface 107 ofthe second III-V compound layer 106. The exposed portion of the secondIII-V compound layer 106 has a substantially flat top surface betweenthe intermetallic compounds 112. The opening 118 is configured as alocation for the later gate electrode formation.

Referring back to FIG. 3, method 300 continues with operation 304. FIG.10 illustrates a cross-sectional view for depositing a p-type layer 120on a portion of the second III-V compound layer 106 between the sourcefeature and the drain feature.

The p-type layer 120 is deposited on the protection layer 114, along aninterior surface of the combined opening 118 and on the exposed portionof the second III-V compound layer 106. The p-type layer 120 is alsodeposited over the source/drain features. The p-type layer 120 has athickness range from about 3 nm to about 30 nm. In some examples, thep-type layer 120 comprises certain metal oxides. Examples of metaloxides used for p-type layer 120 include oxides of Ni, Zn, Fe, Sn, Cu,Al, Ga, Sr and mixtures thereof. In some embodiments, the p-type layer120 comprises NiO_(x), ZnO_(x), FeO, SnOx, CuAlO₂, CuGaO₂ or SrCu₂O₂. Xis in a range of about 1 to about 2. In at least one example, the p-typelayer 120 is NiO_(x). A nickel layer is formed by a sputteringdeposition with a nickel target. Then, an oxidation process is performedto convert the nickel layer into NiO_(x). In other embodiments, thep-type layer 120 is formed by an atomic layer deposition (ALD) method ora plasma enhanced chemical vapor deposition (PECVD) method.

The p-type layer 120 contains point defects, for example, ZnO_(x) has Zninterstitials and oxygen vacancies. The point defects generate electronholes and induce p-type conductivity for the p-type layer 120. Thep-type layer 120 elevates the conductance band, E_(c), at the interface(as shown at location 123) of the first III-V compound layer 104 and thesecond III-V compound layer 106 under the combined opening 118 to alevel higher than the Fermi level, E_(f), at the interface of the firstIII-V compound layer and the second III-V compound layer. The electronsin the carrier channel 108 under the combined opening 118 are depleted.Hence, a depletion region 122 in the carrier channel 108 is generated.The HEMT in the structure 100 is converted from a depletion-mode HEMT toan enhanced-mode HEMT. The carrier channel 108 becomes normally-off anda positive gate voltage is applied to turn on the carrier channel 108 ofthis enhanced-mode HEMT.

In at least one embodiment, the p-type layer 120 is also treated with aplurality of dopants including phosphorous (P), P₂O₅, arsenic (As) orZn₃As₂. Next, an annealing process is performed to activate the dopantsin a temperature range from about 600° C. to about 900° C. in a nitrogenenvironment. The dopants within the p-type layer 120 induce the densityof electron holes to a range of about 10¹⁷ to about 10¹⁹ per cm³. Theelectron holes increase p-type layer 120 toward p-type conductivity andfurther deplete the electrons in the depletion region 122 of the carrierchannel 108.

Referring back to FIG. 3, method 300 continues with operation 305. FIG.11 illustrates a gate electrode 124 is formed over the p-type layer 120.

In at least one example, a gate electrode layer is deposited on thep-type layer 120 and overfills the combined opening 118. Lithography andetching processes are performed on the gate electrode layer to definethe gate electrode 124 between the source feature and the drain feature.The p-type layer 120 not covered by the gate electrode 124 is removed toprevent depleting the electrons in the carrier channel 108 outside thegate electrode 124 region. Edges of the p-type layer 120 and the gateelectrode 124 are substantially aligned. In various examples, the gateelectrode layer includes a refractory metal or its compounds, e.g.,titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW), titaniumtungsten nitride (TiWN), tungsten (W) or tungsten nitride (WN). By usingthe refractory metals or compounds, the method 300 can be implemented inthe production line of integrated circuits on silicon substrate. Thecontamination concern due to unsuitable materials on thesilicon-fabrication process is eliminated. In at least another example,the gate electrode layer includes nickel (Ni), gold (Au) or copper (Cu).

FIGS. 12 to 14 are cross-sectional views at various stages ofmanufacture according to various embodiments of the method 300 of FIG.3. Specially, the operation 303 associated with FIGS. 4 to 6 are shownaccording to a different embodiment in FIGS. 12 to 14.

In FIG. 12, openings 109 of the dielectric cap layer 110 are formedthrough suitable photolithographic and etching processes to exposeportions of a top surface 107 of the second III-V compound layer 106.Then, the exposed portions of the second III-V compound layer 106through the openings 109 are removed by a suitable process such asreactive ion etching (RIE) in order to form a recess within each opening109 in the second III-V compound layer 106 (namely AlN layer 106). In atleast one embodiment, the AlN layer 106 is etched with a plasma process,e.g., chlorine (Cl₂) environment. In at least another embodiment, theAlN layer 106 is removed with an argon (Ar) sputtering process. In atleast one example, the recess extends to a depth D from about 10% to100% of a thickness of the second III-V compound layer 106. In at leastanother example, the recess further extends into the first III-Vcompound layer 104 (namely GaN layer 104). A depth of the recess in thesecond III-V compound layer 106 and the first III-V compound layer 104is from about 100% to 190% of a thickness of the second III-V compoundlayer 106. It is believed that the recess etching process on the secondIII-V compound layer 106 in the plasma environment creates nitrogen (N)vacancies in the second III-V compound layer 106 and the first III-Vcompound layer 104. The N vacancies increase carries so that theelectrical performances for the device are improved.

In FIG. 13, a metal layer is deposited over the dielectric cap layer110, disposed over the interior surface of the openings 109 and therecesses, and contacts a bottom surface of the recess. A photoresistlayer (not shown) is formed over the metal layer and developed to form afeature over the openings 109. The metal layer not covered by thefeature of the photoresist layer is removed by a reactive ion etch (RIE)process that etches the exposed portions of the metal layer down to theunderlying the dielectric cap layer 110. Metal features 111 aregenerated by etching process. The photoresist layer is removed after theformation of the metal features 111. The metal features 111 are at leastpartially embedded in the recess of the second III-V compound layer 106and the dielectric cap layer 110.

In FIG. 14, a thermal annealing process is performed on the metalfeatures 111, such that each metal feature 111, the second III-Vcompound layer 106 and the first III-V compound layer 104 react to forman intermetallic compound 112. Advantageously, the recess formationreduces the remaining thickness of the second III-V compound layer 106after the recess etching process. In some embodiments, the metalelements in the intermetallic compound 112 formed in this annealingprocess diffuse deeper into the second III-V compound layer 106 and thefirst III-V compound layer 104. In some embodiments, the intermetalliccompound 112 improves electrical connection and forms ohmic contactsbetween the source/drain features and the carrier channel 108.

Various embodiments of the present disclosure are used to improve theperformance of a semiconductor structure having a high electron mobilitytransistor (HEMT). For example, in conventional methods, a portion ofthe second III-V compound layer 106 is etched to form a recess for thegate formation of an enhanced-mode HEMT. During etching the recess, theetching uniformity among the semiconductor chip regions on the samesubstrate 102 is hard to control. The electrical performances of eachHEMT in the same semiconductor chip region or the same substrate 102 isnot accurately controlled. In this disclosure, the p-type layer 120 onthe second III-V compound layer 106 depletes the electrons in thecarrier channel 108 for an enhanced-mode HEMT. The p-type layer 120 ineach opening 118 among the semiconductor chip regions on the samesubstrate 102 is uniformly formed. The p-type layer 120 eliminates thedrawbacks in conventional methods. The p-type layer 120 also acts asgate insulator to provide a lower gate leakage of the HEMT in thestructure 100. The intermetallic compound 112 is free of Au andcomprises Al, Ti or Cu. Without using Au in the intermetallic compound112, the method 300 is implemented in the production line of integratedcircuits on silicon substrate, because the contamination concern from Auon the silicon-Fab process is eliminated. Compared with the HEMT havingAu in source/drain feature, the cost for manufacturing the HEMTaccording to the present application is reduced. Both the III-Vsemiconductor compounds process and the silicon-fabrication process areimplemented in the same production line, which increases the flexibilityto allocate different products for the production line.

One aspect of this disclosure describes a high electron mobilitytransistor (HEMT). The HEMT includes a first III-V compound layer. Asecond III-V compound layer is disposed on the first III-V compoundlayer and is different from the first III-V compound layer incomposition. A carrier channel is located between the first III-Vcompound layer and the second III-V compound layer. A source feature anda drain feature are disposed on the second III-V compound layer. Each ofthe source feature and the drain feature comprises a correspondingintermetallic compound at least partially embedded in the second III-Vcompound layer. Each intermetallic compound is free of Au and comprisesAl, Ti or Cu. A p-type layer is disposed on a portion of the secondIII-V compound layer between the source feature and the drain feature. Agate electrode is disposed on the p-type layer. A depletion region isdisposed in the carrier channel and under the gate electrode.

Another aspect of this disclosure describes a high electron mobilitytransistor (HEMT). The HEMT includes a gallium nitride (GaN) layerdisposed on a substrate. An aluminum nitride (AlN) layer disposed on theGaN layer. A source feature and a drain feature are spaced apart anddisposed on the AlN layer. Each of the source feature and the drainfeature includes a corresponding intermetallic compound. Eachintermetallic compound has a non-flat top surface and at least partiallyembedded in the AlN layer. A portion of a p-type layer is disposed onthe AlN layer. A gate electrode is disposed on the portion of the p-typelayer.

The present disclosure also describes an aspect of a method of forming ahigh electron mobility transistor (HEMT). The method includesepitaxially growing a second III-V compound layer on a first III-Vcompound layer. A carrier channel is located between the first III-Vcompound layer and the second III-V compound layer. Two metal featuresspaced apart are formed on the second III-V compound layer. Each metalfeatures is free of Au and comprises Al, Ti or Cu. The two metalfeatures are annealed to form corresponding intermetallic compounds. Ap-type layer is deposited on a portion of the second III-V compoundlayer between the intermetallic compounds. A gate electrode is formed onthe portion of the p-type layer.

Although the embodiments and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A high electron mobility transistor (HEMT)comprising: a first III-V compound layer; a second III-V compound layerdisposed on the first III-V compound layer and different from the firstIII-V compound layer in composition, wherein a carrier channel islocated between the first III-V compound layer and the second III-Vcompound layer; a source feature and a drain feature disposed on thesecond III-V compound layer, each of the source feature and the drainfeature comprising a corresponding intermetallic compound at leastpartially embedded in the second III-V compound layer, wherein eachintermetallic compound is free of Au and comprises Al, Ti or Cu; ap-type layer disposed on a portion of the second III-V compound layerbetween the source feature and the drain feature; a gate electrodedisposed on the p-type layer; and a depletion region disposed in thecarrier channel and under the gate electrode.
 2. The HEMT of claim 1,wherein each intermetallic compound is embedded in the second III-Vcompound layer and a top portion of the first III-V compound layer. 3.The HEMT of claim 1, wherein each intermetallic compound has a non-flattop surface.
 4. The HEMT of claim, wherein each intermetallic compoundis configured to contact the carrier channel.
 5. The HEMT of claim 1,wherein the p-type layer is configured to deplete the carrier channelwithin the depletion region.
 6. The HEMT of claim 1, wherein the p-typelayer is configured to elevate a conductance band E_(c) at an interfaceof the first III-V compound layer and the second III-V compound layer toa level higher than a Fermi level E_(f) at the interface of the firstIII-V compound layer and the second III-V compound layer.
 7. The HEMT ofclaim 1, wherein the p-type layer comprises at least one metal oxide andhas p-type conductivity.
 8. The HEMT of claim 1, wherein the p-typelayer comprises at least one oxide of Ni, Zn, Fe, Sn, Cu, Al, Ga, Sr andmixtures thereof.
 9. The HEMT of claim 1, wherein the p-type layercomprises a plurality of dopants including phosphorous (P), P₂O₅,arsenic (As) or Zn₃As₂.
 10. The HEMT of claim 1, wherein the gateelectrode comprises titanium (Ti), titanium nitride (TiN), titaniumtungsten (TiW), titanium tungsten nitride (TiWN), tungsten (W) ortungsten nitride (WN).
 11. The HEMT of claim 1 further comprising adoped isolation region, wherein the doped isolation region comprisesoxygen or nitrogen.
 12. A high electron mobility transistor (HEMT)comprising: a gallium nitride (GaN) layer disposed on a substrate; analuminum nitride (AlN) layer disposed on the GaN layer; a source featureand a drain feature spaced apart and disposed on the AlN layer, each ofthe source feature and the drain feature including a correspondingintermetallic compound, wherein each intermetallic compound has anon-flat top surface and is at least partially embedded in the AlNlayer; a portion of a p-type layer disposed on the AlN layer; and a gateelectrode disposed on the portion of the p-type layer.
 13. The HEMT ofclaim 12, wherein each intermetallic compound has a top width and abottom width, the top width is wider than the bottom width.
 14. The HEMTof claim 12, wherein each intermetallic compound is free of Au andcomprises Al, Ti or Cu.
 15. The HEMT of claim 11, wherein a carrierchannel is located between the GaN layer and the AlN layer, the carrierchannel comprising a depletion region under the gate electrode.
 16. TheHEMT of claim 11, wherein the p-type layer comprises at least one metaloxide and has p-type conductivity.
 17. The HEMT of claim 11, wherein thep-type layer comprises at least one oxide of Ni, Zn, Fe, Sn, Cu, Al, Ga,Sr and mixtures thereof.
 18. The HEMT of claim 11, wherein the p-typelayer comprises NiO_(x), ZnO_(x), FeO_(x), SnOx, CuAlO₂, CuGaO₂ orSrCu₂O₂.
 19. The HEMT of claim 11, wherein the p-type layer furthercomprises a plurality of dopants including phosphorous (P), P₂O₅,arsenic (As) or Zn₃As₂.
 20. A method of forming a high electron mobilitytransistor (HEMT), the method comprising: epitaxially growing a secondIII-V compound layer on a first III-V compound layer, wherein a carrierchannel is located between the first III-V compound layer and the secondIII-V compound layer; forming two metal features spaced apart anddisposed on the second III-V compound layer, wherein each metal featuresis free of Au and comprises Al, Ti or Cu; annealing the two metalfeatures to form corresponding intermetallic compounds; depositing ap-type layer on a portion of the second III-V compound layer between theintermetallic compounds; and forming a gate electrode on the portion ofthe p-type layer.